The present invention relates to the art of molds for forming ingots from molten metal and, more particularly, to a method and mold plug composition for plugging the opening in the bottom of a top pouring ingot mold.
Top pouring, bottom plugged ingot molds are well known in the art of metal casting. Such molds are characterized by having an opening through the bottom wall thereof which is closed by a removable plug prior to the pouring of molten metal into the top of the mold to form an ingot conforming with the mold shape which, as is also well known, can be of circular or noncircular cross-section transverse to the vertical axis of the mold. Following the casting operation, the plug is destructable to facilitate removal of the ingot from the mold by impacting a tool against the bottom of the ingot through the opening in the bottom wall of the mold.
Plugs for such ingot molds are sometimes provided as preformed ceramic or carbon elements which are introduced into the plug openings prior to the pouring of molten metal into the mold. While such preformed plugs are suitable for new molds having clean and uniform surfaces both on the bottom wall of the mold and in the plug opening through the bottom wall, they become progressively less suitable in connection with repeated use of the mold in that the bottom surface and plug opening in the mold become irregular and progressively more irregular through repeated use of the mold. Moreover, the preforming required in connection with such plugs is time consuming and expensive, and the expense thereof is promoted by the fact that the plugs are subject to breakage during removal thereof from their mold as well as during storage and introduction thereof into a mold plug opening. Such potential breakage requires care with respect to introducing the plug into the opening of an ingot mold. Furthermore, introduction of the plug into a hot ingot mold requires the use of a special tool to protect workmen from injury.
Efforts have also been made heretofore to provide a particulate material curable in situ in the plug opening of a top pouring ingot mold to plug the opening during an ingot casting operation. Such material heretofore provided includes an aggregate of suitable plug forming material and a binder which is admixed therewith and which composition is introduced into the mold plug opening so as to form a plug therein upon curing. The advantages of forming an ingot mold plug in situ as opposed to using a preformed plug include the elimination of the preforming procedure and the equipment required in connection therewith, and eliminating the requirement for care in handling to avoid loss of the plug prior to use such as through breakage. Heretofore, however, efforts to provide a plug material curable in situ have resulted in a number of problems and disadvantages which have limited acceptability thereof and, in connection with specific requirements of certain customers, has rendered the material unacceptable thereto. More particularly in this respect, for example, the materials heretofore used as a binder such as lignin sulfonate and sodium silicate have a high moisture content which has resulted in the mold plug having a moisture content of a magnitude sufficient to promote explosion of the plug and/or molten metal from the mold due to water-steam expansion. Accordingly, it will be appreciated that there has been considerable concern and reluctance to use such material from the standpoint of safety to personnel working therewith. Furthermore, such plug materials heretofore provided have been hydroscopic whereby, if there is a long period of time between plugging and the pouring of molten metal into the mold, the plug material absorbs moisture to promote or further promote the foregoing hazardous condition.
Another disadvantage in connection with curable plug materials heretofore provided resides in the inability to obtain a sufficiently strong plug to avoid contamination of the ingot metal by the plug aggregate, whereby the plug materials or the use of certain aggregates therein have been unacceptable in situations where a high degree of purity is required with respect to the ingot metal. More particularly with respect to this problem, impurities from the plug material may either enter the molten metal poured into the mold or adhere to and thus cause surface contamination of the ingot at the interface between the plug and the ingot. In the first instance, inclusions in the ingot contaminate the metal and thus down grade the ingot, and in the second instance contamination of the ingot at the end adjacent the plug requires cropping to remove the contamination and thus a loss of ingot metal. The existence of contaminating inclusions in the body of the ingot cannot be determined until the ingot is used, and the mere fact that such contamination is possible often results in a refusal to use such plug materials heretofore available.
A further disadvantage attendant to the use of curable particulate mold plug material heretofore available is the inability to sufficiently pack the material in the ingot mold hole to adequately seal the hole against the leakage of molten metal across the plug. Such leakage not only results in loss of production time but also promotes irregularity of the surface of the plug opening which reduces the useful life of the mold. Still another disadvantage resides in the fact that there has been no ability to control the curing of the plug material so as to optimize use of the plug material independent of mold temperature conditions and/or the state of cure of the material at the time molten metal is poured into the mold while, at the same time, optimizing the strength of the cured plug and minimizing the time lapse between introduction of the plug material into the mold and the time at which molten metal can then be poured into the mold.